The invention relates to an aqueous dispersion of a polymer, which polymer has been obtained by subjecting a starting polymer which contains maleic anhydride monomer units and vinyl aromatic monomer units to an imidization reaction. The invention also relates to a process for the preparation of the dispersion, to the use of the dispersion in a paper-sizing composition, and to the use of the paper-sizing composition for the production of paper.
Aqueous dispersions of a polymer, which polymer has been obtained by subjecting a starting polymer which contains maleic anhydride monomer units and vinyl aromatic monomer units to an imidization reaction, are known from DE-A-1720746. The polymer in the known dispersions is obtained from a copolymer of styrene and maleic anhydride, and after it has been subjected to the imidization reaction it contains no more, or virtually no more, maleic anhydride monomer units.
A drawback of the known dispersions is that use of these dispersions in a paper-sizing composition does not lead to a paper-sizing composition that adequately reduces the water-absorption-property of paper, and furthermore use of this paper-sizing composition does not result in paper with good ink-jet printing properties, such as, for instance, wicking, bleeding and blackness. The invention aims to provide a dispersion that does not have said drawbacks.
Surprisingly, this aim is achieved in that the starting polymer contains 7-50 mole % of maleic anhydride monomer units and the imidization reaction has been carried out in such a way that at most 75% of the monomer units of maleic anhydride has been imidized.
The dispersion according to the invention can very well be used to obtain a paper-sizing composition that sufficiently reduces the water-absorption property of paper, and in addition use of this composition results in paper with good ink-jet printing properties. The paper-sizing compositions according to the invention are particularly suitable for surface sizing. A paper-sizing composition containing the dispersion according to the invention also exhibits good foaming behaviour, i.e. little foaming occurs during use of the composition.
It is noted that the German patent specification DE-A-4342157 also discloses an aqueous dispersion with good properties for use in paper-sizing compositions. The dispersion disclosed in said patent specification is prepared by imidizing maleic anhydride monomer units (MA) in a styrene-maleic anhydride copolymer (SMA) to at least 90% with a mixture containing various amines, amino alcohol and a volatile organic acid. According to the introduction to the specification of said patent this mixture is needed to impart good paper-sizing properties to the dispersion. A drawback of these known dispersions is that relatively many different and rather specific chemicals are needed for their preparation, and that there are highly specific requirements as to the composition.
Dispersion is here understood to be a continuous liquid phase with discrete polymer particles.
In a preferred embodiment of the dispersion according to the invention 50-70% of the maleic anhydride monomer units of the starting polymer has been imidized.
The starting polymer used to prepare the dispersion contains maleic anhydride (MA) monomer units and vinyl aromatic monomer units. Suitable vinyl aromatic monomer units are, for instance, styrene and alpha-methylstyrene. The starting polymer preferably contains styrene monomer units. Besides monomer units MA and vinyl aromatic monomer units, the starting polymer may contain other monomer units.
A copolymer containing vinyl aromatic monomer units and MA monomer units can be prepared using the known processes, for instance using the process described for the preparation of a copolymer of maleic anhydride and styrene monomer units, by Hanson and Zimmerman, Ind. Eng. Chem., vol. 49, No. 11 (1957), pp. 1803-1807. Preferably, the starting polymer contains 18-50 mole % maleic anhydride monomer units and 50-82 mole % styrene monomer units. More preferably, the starting polymer contains 22-36 mole % maleic anhydride monomer units and 64-78 mole % styrene monomer units. The molar weight of the starting polymer may vary within wide limits. Preferably, a starting polymer has a weight-average molecular weight of between 50,000 and 180,000 kg/kmol.
In a possible process for the preparation of an aqueous dispersion according to the invention the starting polymer is introduced into an autoclave together with an aqueous solution of NH3 or an amine, the temperature being at least 95xc2x0 C., the molar ratio of maleic anhydride monomer units and NH3, or maleic anhydride and amine, being between 1:0.8 and 1:5, and the pressure being so high that the aqueous solution does not start to boil. The time needed for at most 75% imidization of the maleic anhydride monomer units in the starting polymer depends on the chosen temperature, the reactor, the amounts started from, and the like, and can readily be determined by experiment. The imidization reaction can be stopped, or at any rate be strongly slowed down, by reducing the temperature to below 95xc2x0 C.
In a preferred embodiment of the process, the temperature is between 100 and 130xc2x0 C. since at that temperature a dispersion that is very well defined in terms of properties and composition can be obtained with a high reproducibility. The molar ratio of maleic anhydride monomer units and NH3 or maleic anhydride and amine is preferably between 1:1.5 and 1:2.5. Preferably, use is made of ammonia or an aliphatic amine, which aliphatic amine is a primary amine. Examples of suitable amines are butylamine and stearylamine.
It is possible to add an emulsifier to the reaction mixture. Addition of an emulsifier promotes the formation of the dispersion according to the invention.
Examples of suitable emulsifiers are the sodium salt of (C10-C13) alkyl-benzene sulphonic acid, stearyl dimethylbenzyl ammonium chloride and ethylene oxide-propylene oxide copolymers.
The dispersion according to the invention is very suitable for forming part of paper-sizing compositions. Besides a dispersion according to the invention paper-sizing compositions contain one or more of the usual additives. Preferably, starch is used as an additive. The amount of dispersion contained in the composition is so large that the amount of polymer, i.e. the dry weight of polymer particles in the dispersion, is for instance between 0.2 and 10 wt. %, relative to the dry weight of the other customary additives. Preferably, a paper-sizing composition contains between 2 and 6 wt. % of dry weight of polymer particles, relative to the dry weight of starch. A composition can for instance be prepared by adding a certain amount of the dispersion according to the invention to a solution of the other additives in water.